Delivering a hastily prepared statement to reporters, Obama said the U.S. is “deeply concerned” by the Russian troop movements in the majority ethnic-Russian peninsula. “The United States will stand with the international community in affirming there will be will be costs for any military intervention in Ukraine,” he said.

The Russian military action comes days after ousted Ukrainian President Viktor Yanukovych fled to Russia following months of protests and government crackdowns that turned violent. Obama announced that Vice President Joe Biden, the Administration’s point person on Ukraine, spoke with newly selected Prime Minister Arseniy Yatsenyuk to express the U.S. government’s support for Ukraine’s territorial integrity.

“Any violation of Ukraine’s sovereignty and territorial integrity would be deeply destabilizing, which is not in the interests of Ukraine, Russia or Europe,” Obama said. “It would represent a profound interference in matters that must be determined by the Ukrainian people. It would be a clear violation of Russia’s commitment to respect the independence and sovereignty and borders of Ukraine, and of international laws.”

Although Obama vowed consequences for Russian aggression, he has limited means of pressuring President Vladimir Putin, whose cooperation he badly needs on other issues, including Iran’s nuclear program and the Syrian civil war.

Obama’s comments follow warnings by Secretary of State John Kerry and national-security adviser Susan Rice that Russian President Vladimir Putin would be making a “grave mistake” by intervening in Ukraine. Those officials never specified what penalties Russia might face, perhaps because U.S. options are so limited: America has few direct economic ties to Russia. Moscow can use its U.N. Security Council seat to veto any U.N. resolution that might seek to condemn or punish its actions. And the prospect of a U.S. military involvement is virtually unimaginable.